The notion of ‘simultaneity’ is at the core of the concept of global time in physics, manifest in the physical theories of relativity, the Lorentz Transformations, synchronization of clocks, astronomy and cosmology, physical metrology, precision navigation using atomic clocks etc. The foundations that determine the current theories and usage of physical time and simultaneity were laid by H. Poincaré and A. Einstein. How do we understand ‘simultaneity’ physically, and how does it differ from our intuitive understanding? What is the operational meaning and significance of ‘simultaneity’ for relativity, globally synchronized time, and the propagation of signals? I will explore the many questions to carefully excavate till we reach the true foundation and understanding.